1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed on the electroconductive support, comprising a charge generating material and a charge transporting material consisting of an .alpha.-cyanostilbene compound.
2. Discussion of Background
Conventionally, an inorganic photoconductive material such as selenium, selenium-tellurium alloy, or zinc oxide has been widely used for a photoconductive layer of an electrophotographic photoconductor. Recently, studies of electrophotographic photoconductors comprising organic photoconductive materials are made, and some of them are used in practice. Of the above-mentioned organic electrophotographic photoconductors, laminate-type electrophotographic photoconductors are mostly put to practical use, in which photoconductors a photoconductive layer comprises a charge generation layer and a charge transport layer. Owing to such a function-separating photoconductive layer, there is improvement in the photosensitivity and the life span of the photoconductor comprising the organic material, which are conventionally regarded as disadvantageous as compared with the photoconductor comprising the inorganic material. Therefore, the electrophotographic photoconductors comprising the organic photoconductive material have been actively developed, with taking full advantage of the merits of the organic photoconductive material, that is, low cost, safety and diversity.
The above-mentioned laminate-type electrophotographic photoconductor generally comprises an electroconductive support, and a charge generation layer comprising a charge generating material such as a pigment or a dye, and a charge transport layer comprising a charge transporting material such as hydrazone or pyrazoline, which layers are successively overlaid on the electroconductive support. Because the charge transporting material contained in the charge transport layer is an electron-donor material, the above-mentioned photoconductor is of positive-hole-transfer type. Therefore, when the surface of the photoconductor is negatively charged, it exhibits the photosensitivity. However, corona discharge is unstable when used for negatively charging the photoconductor as compared with when used for positively charging the same. The amount of ozone or nitrogen oxides generated in the course of negative charging by the corona discharge is about 10 times that generated in the course of positive charging. Those products are attached to the surface of the photoconductor, so that the photoconductor physically and chemically deteriorates. Furthermore, those products cause the environmental problem.
In addition to the above-mentioned problems, to carry out the development of the negatively-chargeable photoconductor, a positive toner is necessitated. However, from the viewpoint of a triboelectric series, it is difficult to produce such a toner having a positive polarity as can be used with ferromagnetic carrier particles. In a two-component high-resistivity magnetic brush development method, therefore, negatively-chargeable toner is more stable and can be more freely selected and used as compared with the positively-chargeable toner. In this regard, a positively-chargeable photoconductor is more advantageous and can be more widely used than the negatively-chargeable photoconductor.
With the above-mentioned advantages of the positively-chargeable photoconductor taken into consideration, there are proposed positively-chargeable photoconductors comprising the organic photoconductive materials. For example, it is proposed that a charge transporting material having a high electron-transporting capability such as 2,4,7-trinitro-9-fluorenone be contained in a charge transport layer when the photoconductor is formed by laminating a charge transport layer on a charge generation layer. However, this compound has carcinogenicity, so that the above-mentioned photoconductor is extremely unsuitable for use in practice from the viewpoint of industrial hygiene.
Moreover, U.S. Pat. No. 3,615,414 discloses a positively-chargeable photoconductor comprising a thiapyrylium salt serving as a charge generating material which forms an eutectic complex by combining with polycarbonate serving as a binder resin. However, the above-mentioned photoconductor has the shortcoming that a memory phenomenon tends to occur, and therefore ghost images are easily obtained.
It is possible to prepare a positively-chargeable photoconductor with a laminate-type photoconductive layer in such a configuration that a charge generation layer comprising a charge generating material which can generate a positive hole or electron when the photoconductor is exposed to light, is formed on a charge transport layer which contains a charge transporting material capable of transporting the positive-hole or electron. However, in the above-mentioned structure of the positively-chargeable photoconductor, the charge generation layer becomes a surface layer, so that the charge generating material, which is fragile to external influences, for example, the application of short wavelength light such as ultraviolet light for light exposure, corona discharge, humidity, mechanical friction, inevitably exists in the surface portions of the photoconductor. As a result, electrophotographic properties deteriorate during the preservation of the photoconductor and the image formation process. Consequently, the quality of the obtained images is decreased.
On the other hand, the conventional negatively-chargeable photoconductor comprising a charge transport layer serving as a surface layer formed on a charge generation layer is scarcely subjected to the above-mentioned external influences. Rather, the charge transport layer formed on the charge generation layer has an effect of protecting the charge generation layer.
In the positively-chargeable photoconductor, it is proposed to provide on a charge generation layer comprising a charge generating material a thin protective layer comprising, for example, an insulating, transparent resin to protect the charge generation layer from the aforementioned external influences. However, when the photoconductor is exposed to light, there is a difficulty in efficiently generating electric charges in the charge generation layer because the light applied to the charge generation layer is intercepted by the protective layer. Therefore, the effect of light application is decreased. Furthermore, in the case where the thickness of the protective layer is large, the photosensitivity of the photoconductor deteriorates.
As mentioned above, various proposals for obtaining the positively-chargeable photoconductors have been made, but they have many problems with regard to the photosensitivity, the memory phenomenon or the industrial hygiene.